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When asked whether it could go all the way to Strasbourg, she replied: “Of course. I don’t know if that will happen but it is possible.”

He’s prepared to take whatever steps are necessary to protect his freedom of speech and his reputation

A source close to Mr Amaral

The couple sued Mr Amaral over claims he made in his book, The Truth Of The Lie. He was forced to take early retirement while investigating Madeleine’s disappearance in Praia da Luz on the Algarve nine years ago.

Last week he won a significant victory in his efforts to restore his reputation when Portugal’s Appeal Court found in his favour.

The decision overturned a lower court’s ruling ordering him to pay the McCanns almost £400,000 for alleged libel over claims he made in his book, that Madeleine had died in apartment 5a of the Ocean Club in May 2007.

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Goncalo Amaral is a former Portuguese police chief

He has insisted the theory came from information in the original police files. He also claimed he had the right to express his opinion on the case after being removed from the investigation in October 2007.

Ms Duarte has been instructed by the McCanns to fight the appeal court’s judgment at the Supreme Court in Lisbon.

If Madeleine’s parents lose this challenge they face a double legal whammy as Mr Amaral has signalled his intention to sue them for damages to his reputation, potentially putting at risk some of the £746,152 in the McCann’s Maddie Fund.

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Goncalo Amaral is being sued by the McCanns by over claims he made in his book

If it goes to the European Court of Human Rights, the legal bills could be astronomical. The court encourages parties to reach a “friendly settlement” but a lengthy and costly hearing seems likely.

A source close to Mr Amaral said: “He’s prepared to take whatever steps are necessary to protect his freedom of speech and his reputation.”

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Mr Amaral was order to pay the McCanns almost £400,000 over libel claims

Meanwhile, copies of his book are expected to go back on sale in Portugal this week. The McCanns said in a statement last week that they were “disappointed” by the ruling from the Appellate Court.

The statement added: “We will continue to defend ourselves against this libel and the search for our daughter.